3.Public Key Distribution System

Chapter 3

Public Key Distribution System

In a communication network, two users, wishing to communicate with each other via a common-key encryption scheme, have to share a secret cryptographic key not known to

other users. This is what we call the key distribution problem.

This chapter considers key establishment protocols and related cryptographic tech-

niques which provide shared secrets between to parties, typically for subsequent use as

symmetric keys for a variety of cryptographic purposes including encryption, decryption,

and message authentication. Remainder of this chapter is organized as follows. Section 1

provides background materials including basic definitions and concepts, and discussion of

objective. Section 2 and section 3 discuss one-way functions and key agreement proto cols,

respectively, based on Diffie-Hellman Key Exchange Protocols.

3.1 Definitions

The purpose of a cryptosystem is to encipher an intelligible cleartext (also called plain-

text), thus producing an unintelligible ciphertext (also called cryptogram). The included

receiver must be able to decipher the ciphertext, thus recovering the plaintext. However,

eavesdroppers (also called crptanalysts) must be unable to decrypt the ciphertext. Notice

the important difference between deciphering and decryption.

There are several ways in which cryptosystems can be classified. Generally, cryptosys-

tems are classified into two classes; Private-Key cryptosystems which are beyond this

research, and Public-Key cryptosystems which will be discussed in whole this research.

A cryptosystem is a five-tuple (M, C, K, E , D), where the following conditions are

satisfied:

•M denotes a set called the message space. An element of M is called a plaintext

message or simply a plaintext.

•C denotes a set called the ciphertext space. An element of C is called a ciphertext.

•K denotes a set called the key space. An element of K is called a key.

•E denotes a set called the enciphering transformation,